Okay anybody whish to argue against these points: 1) closed or open is the same if no instances overlap 2) overlapping instances (open or closed) can break code in modules which import the defining module if a new instance is declared in any imported module. 3) code changes in non-imported modules cannot have any affect on _this_ module. (And here I count any module in the import tree as imported - at least in terms of recompilation dependancies) The conclusion appears to be it is overlapping instances that cause code to be 'breakable' by simply defining a new instance and not closing the class. Closing the class would appear to have no adverse affects on existing programs (as it allows better improvement rules) all existing programs that compile without the better improvement rules should still compile - just a few more programs will be valid with the closed assumption? Keean.